1. Field of the Invention
This invention relates to fuel cells and more particularly to electrolyte retaining matrices and methods for making the same.
2. Description of the Prior Art
Fuel cells for the production of electrical energy from a fuel and oxidant are well known in the art. Such cells, in their most simplified design, comprise a housing, an oxidizing electrode spaced apart from a fuel electrode, and an electrolyte disposed between and in contact with said electrodes. The electrolyte can be a solid, a molten paste, a free-flowing liquid, or a liquid trapped in a matrix. This application is concerned with the latter type of matrix which is preferred for many applications.
For optimum performance in a fuel cell employing a trapped aqueous electrolyte, the matrix must exhibit certain properties. For example, the matrix must be hydrophilic. Also, it must be continuous to prevent gas crossover or mixing of reactant gases in the fuel cell; in other words, it should be entirely free from pin holes and cracks. It should be as thin as possible in order that the internal resistance losses through the electrolyte will be minimal. Intimate contact between the matrix and electrode surface is necessary to maximize catalyst utilization. Uniform thickness is also critical to good performance in that lack of uniformity can cause current maldistributions with a loss in performance. It is also desirable that the pore size distribution of the matrix be very well controlled so as to prevent gas crossover and to insure proper electrolyte distribution throughout the cell.
Compounding the problems of achieving the foregoing properties is the fact that one is limited in the choice of materials which can be used. For example, the materials must be chemically and thermally stable at cell operation temperatures; also, they must not poison the catalyst and they must have high electronic resistance. Finally, the matrix should be made by an economical process.
A common prior art economical method for making matrices has been by paper making techniques, wherein the matrix is formed into a sheet and sandwiched between the electrodes in a fuel cell or fuel cell stack by mechanical means. For example, Landi U.S. Pat. No. 3,407,249 forms sheets of fibrillated polytetrafluoroethylene. Mesite et al U.S. Pat. No. 3,627,859 forms a matrix sheet from cellulosic fibers in combination with a fluorocarbon polymer. Emanuelson et al U.S. Pat. No. 3,694,310 forms mats of matrix material from phenolic resin fibers coated with a phenolic beater addition resin.
Regardless of the material from which the mat is made, the mechanical sandwiching of a sheet type matrix between electrodes is deficient in that it does not necessarily result in intimate contact between the matrix and the electrode over the entire surface of the matrix. A further problem with making matrices by paper making techniques is that the desired thinness cannot be achieved without losing the property which prevents gas crossover. Even if the matrix sheet could be made as thin as desirable, it would be extremely difficult, if not impossible, to handle.
Another method for forming a matrix, which overcomes some of the problems with the paper making techniques, is to form the matrix directly on the surface of the electrode such as by dipping the electrode into an aqueous solution of the matrix material as described in Blanc et al 3,022,244. This has also been accomplished by spraying or painting the matrix onto the surface of the electrode. While these techniques overcome some of the handling problems associated with separate matrix sheets, it is difficult to maintain a uniform thickness. Because of the non-uniformity of the thickness it may be necessary that some areas be thicker than desirable in order to assure that there are no bare spots in the thinnest areas.
Despite the fact that those skilled in the art know what properties and characteristics are necessary for a high performance, fuel cell matrix, it is apparent from the foregoing that a fully satisfactory method for producing such a matrix in an economical manner has not been discovered.
A well-known technique for applying thin layers of various materials onto a variety of substrates is the screen printing or silk screening process. Lee et al U.S. Pat. No. 2,779,975 discusses the use of screen printing for manufacturing electrical components such as capacitors, inductors, resistors, printed circuits, and the like, each of which is composed of built-up layers arranged in a desired pattern, the layers being electrically connected to one another. Screen printing is also, of course, well known for applying decorative patterns onto surfaces such as described in Thompson U.S. Pat. No. 3,577,915.
Despite the diversified uses known in the art for screen printing, there is no teaching or even suggestion that screen printing be used to apply a matrix material to the surface of an electrode. It is the Applicants' belief that this is because persons skilled in the art did not believe screen printing could do a satisfactory job. This is discussed further in the Summary of the Invention.